1. Field of the Invention
The present invention relates, in general, to air conditioning apparatus. In particular, the invention relates to a multiple type air conditioning apparatus including one external unit (refrigerant supply unit) having a compressor, a heat-exchanger and a fan device, and a plurality of internal units (air conditioning units), each having a heat-exchanger and a fan device, connected in parallel to one another.
2. Description of the Related Art
In a conventional multiple type air conditioning apparatus typically including a common external unit and a plurality of internal units, the capacity of a compressor housed in the external unit is controlled based on the total amount of required heating/cooling capacities of the plurality of internal units. In addition, a flow rate control valve and an expansion valve are provided to the liquid flow pipe (the down-stream side of the heat-exchangers of the internal units in the heating operation) respectively connected to each internal unit, and a degree of opening of each flow rate control valve is controlled according to the required heating/cooling capacity of the corresponding internal units. In particular, in the cooling operation, the refrigerant super heating of each internal unit is maintained at a constant value by controlling the degree of opening of the corresponding expansion valves.
In the above-described conventional multiple type air conditioning apparatus, since the degree of opening of the flow rate control valve of each internal unit is independently controlled, the following problems may occur if a big difference in the required capacity exists among the plurality of internal units in the cooling operation. A sufficient amount of refrigerant flows to one of the plurality of internal units which requests a large capacity. However, it is difficult to control the refrigerant super heating of another internal unit requesting a capacity smaller than that of the one of the plurality of internal units at a constant value. This is because the amount of refrigerant flowing through another internal unit is originally small and a decrease in the refrigerant super heating may be limited, no matter what the degree of opening of the expansion valve of another internal unit is controlled. If the refrigerant super heating is in excess of the constant value, the entire refrigerating circuit becomes unstable and a desirable cooling capacity of the air conditioning apparatus may not be achieved.
On the other hand, in the heating operation, when the refrigerant flow rate control operation by the flow rate control valve is carried out at the down-stream side of each internal unit, a large amount of liquid refrigerant stays in another internal unit which requires a small capacity, and thus the amount of refrigerant circulating through the entire refrigerating circuit is insufficient. As a result, the temperature of refrigerant in the refrigerating circuit decreases and a desirable heating capacity is not obtained.
To solve the above-described problems, it is considered that the volume of refrigerant charged in the refrigerating circuit is increased beforehand. However, a liquid tank having a large capacity is required to store such an increased amount of refrigerant, and thus the external size of the apparatus is also increased.
On the other hand, if a sufficient heating capacity is obtained without increasing the amount of refrigerant, an increase in the rotational speed of the compressing motor is required resulting in an increase in the power consumption of the compressing motor.